Question for Buswarrior, our resident air brake authority.....

[Ncbob]

I've, periodically had a problem releasing the parking brakes of my DD3 system. And, at other tiimes, pushing in the plunger releases them without having to push the treadle. If I apply the parking brakes with the system pressure at 125 PSI (as against using the treadle to drop the pressure lower or to 60 PSI) will the lower pressure make it easier to unlock them when I want to move? I've often thought about bleeding the system via my moisture bleeders and letting them come on automatically...more to test them working properly than to apply them manually.

Input please?

NCbob

[bevans6]

Your 5A may be different than my 5C.  On my bus the parking brake is applied using pressure from the parking brake tank through a 85 PSI pressure regulator.  The pressure in the main system is irrelevant.  The correct way to release the parking brake is with a 5 second 100 PSI full service brake application.  Sometimes just pushing in the valve will release them, but a certain set of circumstances has to be in place for that to work, and it's not going to happen every time.  But your 5A may be plumbed differently in this regard.

To test the automatic application, you have to bleed the pressure in the parking brake tank to below 40 PSI, then the spring should make it pop up on it's own.  There is no gauge on the parking brake tank, so you don't have a great way to know what pressure it is at.

Brian (not BW, but I am working on my brake system skills...)

[buswarrior]

As Brian has noted, there are variables that are blind to the driver that allow for easy release, however, I expect at times, you are having trouble getting a release at all. I am assuming you are using the correct release procedure to no avail.

The condition of the DD3 internals, the pushrod and the rollers, rust, grooves, or otherwise less than smooth conditions, the ring and spring that push the rollers back, and the all important grease zerk on the DD3 that lubes these things are all variables, as well as the actual air pressure being applied to the different sections of the chamber.

Is there a grease zerk on each chamber, and have you put any grease to them? Go easy, if it takes a pump, that'll do for now, too much and it'll leak onto the locking surfaces, and that is bad.

Perhaps next would to confirm that everything pneumatic is the way it is supposed to be?

Some bits and pieces of brass fitings and a pressure gauge or tire gauge, depending on your hardware, will be needed. Usual cautions about safely blocking the coach from movement or settling before going under there.

1) Leaks? Easiest and needs no tools! Under steady conditions, three configurations, parked, released and released/service brake applied, check for air leaking out of the exhaust ports of the relay valve and inversion valve, or the DD3 chambers. There should be none at all. Also a good time to have an eye and ear for any other leaks, since you're under there anyway, check the exhausts on the treadle valve and the quick release valve on the front axle along the way as well... Soapy water in a spray bottle is always a help for leak searching.

2) Is the parking tank properly charged and isolated? Check pressure via a fitting in the drain, then drain the other tanks and confirm the pressure reading in the parking tank doesn't drop. Parking tank should be at system pressures to start, and stay there.  I like to install male shop airline fittings and quarter turn valves in the tank drains, a tire gauge will fit, or a gauge with female air fitting clips right on, as well as being able to access the tanks either for air in or air out.

3) Is the pressure regulator in the DD3 parking circuit regulating? Check applied parking pressure to the chambers by plumbing your gauge to whichever line or fitting or port that you can loosen on the sides of the inversion valve that lead to the chambers or the other ends at the parking or locking ports. Just put the parking control in whichever position will pressurize the portion you tapped into. IIRC, the MC5A might only have 65 lbs on the regulator, instead of the later setting of 85 lbs. However, the problem at hand would manifest itself with extremes, really high or really low.

4) What pressure is being applied to the service portion of the chambers? A recent thread on one of the boards had a busnut unwittingly operating without rear brake application. On a service brake application, do you have a sence that there is a mighty squeeze, a sence of some flexing? Otherwise, loosen one end or the other of the lines to the service portion, middle hose on the chamber, that will come apart, install gauge and step on and hold the brake pedal to see what you get.

5) Inversion valve is how old? Periodically sticking internally? If unknown age, just change it out and call it preventive maintenance. These are notoriously undermaintained by the second/third tier commercial carriers, and then we inherit them.

Let us know!

happy coaching!
buswarrior

[Ncbob]

BW, I relined the rears, replaced the DD3 cannisters and replaced the drums and the return springs 2 years ago and have only logged a little over 1000 miles since then. Since I'll have access to a pit before I leave for FL in a few weeks I'll make it a point to replace the inversion valve and, as you say, chalk it up to good maintainence. Since the reline I believe I've seated the new shoes to the new drums and did a slack adjuster fine tune this spring.

My parking brakes apply at about 65 Psi under normal circumstances..I know because I've pumped the treadle on a couple of occasions before shutting down and felt the valve move about 65 PSI. Was just wondering if they normally apply at that pressure even though the system pressure might be higher and require a longer or more than one depression of the brake treadle to get the released.

As always thanks for the replies and I'll keep a close eye on he whole system. I'd rather not move at all than have a failure under the worst conditions.

NCbob

[zubzub]

The more i learn about DD3s the more I'm glad i have spring brakes.  Dem dare DD3s shur  have lots of bits inside of em.  spring brake adds 2 moving parts to the brake can, and they fail on.

[gumpy]

Bob, as stated, there should be an 85 psi regulator on the parking brake chamber. You should not be making a full application on the brake pedal when you apply the parking brake. Otherwise it could override the 85 psi and set them tighter. It's important to release the button BEFORE making the full application when releasing the parking brake because the higher pressure is needed to
overcome the tension that was caused by the 85 psi when they were set. You need a brake application pressure that is higher than the set pressure to properly release them. You should never make a full pressure application while the parking brakes are applied.

[JackConrad]

The more i learn about DD3s the more I'm glad i have spring brakes.  Dem dare DD3s shur  have lots of bits inside of em.  spring brake adds 2 moving parts to the brake can, and they fail on.

I agree, unfortuanately, there is not enough room to install spring brakes on the MC 7,8, & 9. I do not know about fit-ability on other makes or models.  Jack

[robertglines1]

lack of use and age is a problem in anything.when my air wipers started working funny I removed line and put a shot of air line oil in it and reattached.all was good. did the same thing with parking brake valve.rite or wrong it worked..temp fix I don't know it's only been two years..maybe do same thing to my aching joints   Bob

[bevans6]

If your push/pull valve operates automatically at 65 PSI by fanning the pressure down in the dry tank with the brake pedal, then I would say that something is wrong.  First thing is that the push/pull valve is supposed to be reacting to pressure in the parking brake tank.  The parking brake  tank is supposed to be isolated from the dry tank (and the accessory tank) by a check valve.  If the parking brake tank pressure is going down along with the dry tank pressure, then the check valve probably isn't working and the whole point of the DD3 system having an emergency braking ability is lost.  The parking brake tank has to be separated from the dry tank and has to maintain it's pressure even if the dry tank is at zero.  Second thing is the spring in the PP valve is supposed to set the brakes at 40 psi, but it's possible that an early one might be set at a higher pressure.  BW alluded to 65 psi in his post, so I am not sure about that part.

I would do this:  pressurize the system up to 120 psi, or wherever your compressor is set to release at.  Turn the bus off, chock the wheels.  Do not set the parking brake.  drain the wet tank, then drain the accessory tank, then drain the dry tank by opening their drain valves.  At this point you should have full system pressure in the parking brake tank, and zero in every other tank.  The Push/pull valve should not have tripped, the parking brakes should NOT be set automatically.  Now drain the parking brake tank.  Along the way, the pressure should fall to the point where the PP valve automatically trips and applies the parking brake. 

When you drain the tanks in this sequence you can check a number of check valves and the pressure protection valve.  When you first drain the wet tank, the check valve between it and the dry tank must work and retain full pressure in the dry tank.  When you next drain the accessory tank, the pressure protection valve should operate at around 65 psi and retain pressure in the dry tank even when the accessory tank is completely empty.  At this point the check valve on the parking brake tank should have allowed it to retain full pressure and the dry tank should have around 65  psi in it.  Next, when you drain the dry tank the only tank left with air should be the parking brake tank.  Note that the parking brake tank is also connected directly to the inversion valve - a bad inversion valve could also drain the parking brake tank even if it's check valve is good. 

Brian

Brian

[Ncbob]

And people look odwn their noses at bus convertors? I never cease to be amazed at the amount and level of intelligence contained herein!

NCbob

[RJ]

DD3 Brake Owners -

The proper procedure for SETTING the DD3 parking brake in normal operation is:

1.  Bring coach to a full stop, place transmission in neutral.

2.  With just barely enough pressure on the service brake pedal to keep the coach from rolling, pull the parking brake knob.

3.  Release service brakes - coach should not move.



The proper procedure for RELEASING the DD3 parking brake in normal operation is:

1.  Start engine, bring coach air system up to full governor cut-out pressure (about 120 psi).

2.  With foot only resting on the service brake, push in parking brake knob.

3.  Wait for a second ("One one thousand") with knob in, then apply a FULL service brake application and hold for five seconds: ("One one thousand, Two one thousand, Three one thousand, etc.")

4.  After five seconds, release service brakes, coach should roll.  If not, repeat steps 1 > 3. 

5.  If they do not release after repeating this sequence two or three times, have the system serviced ASAP - something's wrong.


Print this out for future reference??


FWIW & HTH. . .

[buswarrior]

Yes, Bob, a little spray lube is a handy thing for valve life extension, especially if brake alcohol has been employed in the system. Use a brand without solvents.

An area of theory that is blind to the driver, parking control valves will apply at whatever air pressure that the internal spring will overcome.

And that is the pressure local to the valve, which may or may not be tank pressure under dynamic conditions.

Air behaves like water in respect that it washes around in waves, you remove some somewhere, there is a localized low pressure area, and then there will be a wave/wind towards the low pressure area. If the parking control valve is plumbed close to wherever air can be removed, and it gets a sniff of weaker pressure that the internal spring can snap it into the parked position, it will.

Depending on the vehicle, aggressive, fast and deep brake applications may produce quite a different result from slow, light, steady air loss.

Also, the inversion valve has an internal spring that is kept in the released/go for a drive position by air pressure, and the same issues apply.

Whether the inversion valve has already applied the parking portion or the parking control valve does it, when the air pressure is dropping in the parking tank, the driver does not know sitting in the seat while stationary which or what has really happened.

This is similar for spring brakes, the big springs in the chambers are already firmly applied long before the parking control valve automatically applies, but the driver doesn't know it sitting still.

Beware of driver trainers, most of them don't know any of this, and do static tests, blissfully unaware of the true functionality of the system, and its variables.

happy coaching!
buswarrior